Homemade Sextant

Updated February 21, 2017

Centuries before the invention of GPS location devices, mariners and explorers found their way around the globe with the aid of a sextant. A sextant can be used to determine your latitude by taking a reading off Polaris, the North Star. It can also, with the aid of simple geometry, help you determine the height of an object. If you’re eager to try your hand at using a sextant, you can make a serviceable instrument with material you probably have at home.

Thread a piece of string through the centre hole on the flat edge of the protractor. Tie a knot in the string on the backside of the protractor to keep it in place.

Pull the string across the curved edge of the protractor. Use the scissors to cut the string long enough to leave to leave about 2 inches hanging over the curved edge.

Thread the string through the centre of the washer. Knot the string around the washer. The washer acts as a weight to keep the string pointing straight toward the ground while you take your readings.

Tape the drinking straw across the protractor. One end of the straw should cross the centre of the flat edge where the string is secured. The other end of the straw should be centred on the 90-degree mark on the middle of the curved edge.

Locate the North Star. It’s the last star in the handle of the Little Dipper.

Hold your sextant vertically. The string and weight should swing freely against the protractor.

Looking through the straw with the flat edge of the protractor facing you, sight on Polaris.

When the string stops moving, pinch it to the side of the protractor. The reading where the string crosses the edge of the protractor is your latitude.


It might be difficult to keep the sextant perfectly still while sighting on the star. Take three or four readings and average them for a more accurate fix.

Things You'll Need

  • Protractor
  • String
  • 3/4-inch washer
  • Scissors
  • Drinking straw
  • Adhesive tape
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About the Author

Finn McCuhil is a freelance writer based in Northern Michigan. He worked as a reporter and columnist in South Florida before becoming fascinated with computers. After studying programming at University of South Florida, he spent more than 20 years heading up IT departments at three tier-one automotive suppliers. He now builds wooden boats in the north woods.